A.J. Mulder

On/off control in FES-induced standing up: A model study and experiments

Control of paraplegic standing up was studied with respect to the limitation of the end-velocity of the knee joint when the patient reaches the upright position. Closed-loop on/off control of knee extensor muscle was compared with open-loop controlled standing up both in a model study and in paraplegic patients in a controlled model situation. Criteria were knee-end-velocity and knee extensor muscle activation time. Sensitivity of the system to additional arm support and (in the model study) to the dynamics of knee extensor muscle was studied. It is concluded that the control scheme may reduce knee-end velocity to about 40 per cent and knee extensor activation time to near 70 per cent of the respective open-loop values.

Low-level finite state control of knee joint in paraplegic standing

Low-level finite state (locked-unlocked) control is compared with open-loop stimulation of the knee extensor muscles in functional electrical stimulation (FES) induced paraplegic standing. The parameters were: duration of standing, relative torque loss in knee extensor muscles, knee angle stability, average stimulus output and average arm effort during standing. To investigate the impact of external mechanical conditions on controller performance, experiments were performed both under the condition of a freely moving ankle joint and of a mechanically stabilized ankle joint. Finite state control resulted in a 2.5 to 12 times increase of standing duration or in a 1.5 to 5 times decrease of relative torque loss in comparison with open-loop stimulation. Finite state control induced a limit cycle oscillation in the knee joint. Average maximum knee flexion was 6.2 degrees without ankle bracing, and half that value with ankle bracing. Average arm support was 13.9 and 7.5% of the body weight without and with ankle bracing respectively.

Vibrotactile stimulation of the upper leg: Effects of location, stimulation method and habituation

In this study vibrotactile stimulation of the upper leg and its usability for feedback was tested. Three experiments were performed on ten healthy subjects using pager motors. The first experiment was to test the perception of the vibration at different frequencies and at different locations of the upper leg. The second experiment tested the ability of subjects to estimate location and number of stimuli in an array. In addition it was evaluated whether simultaneous or sequential stimulation is better interpretable. Thirdly the habituation of the vibration was determined. The experiments showed that vibrotactile stimulation is well perceived and can be useful in providing feedback on the upper leg. Further experiments are needed to determine the effectiveness of vibrotactile stimulation for feedback in trans-femoral prostheses.